Shower Pan Mortar Bed: Pre-Slope Thickness and Volume
How thick a shower pan's mortar bed is at the walls, how much mud it takes and how many bags to buy.
Example
You enter
- Pan length (ft) 5
- Pan width (ft) 3
- Drain distance from one end wall (ft, blank = centered) 2.5
- Drain distance from one side wall (ft, blank = centered) 1.5
- Bed thickness at the drain (in) 0.75
- Pitch at the farthest wall (in per ft, 1/4 minimum) 0.25
- Yield per bag (cu ft, from the bag) 0.45
You get
- Rim thickness (in) 1.375
- Average thickness (in) 1.16667
- Volume (ft³) 1.45833
- Bags 4
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeThe perimeter is screeded level and the drain low, so the bed is a shallow pyramid of flat facets: the farthest wall sets the rim at the minimum pitch toward the drain, 1/4 in per ft (the liner pitch of IPC 2027 421.5.2), and the nearer walls run steeper. Each facet averages a third of the drain thickness plus two-thirds of the rim, so the whole bed does. A 5 by 3 ft pan with a centered drain and 3/4 in of mud at the drain has a 1.375 in rim, averages 1.17 in, and takes 1.46 cu ft, 4 bags at 0.45 cu ft; a 6 by 4 ft pan with the drain 1.5 ft from one end and 1 in at the drain has a 2.125 in rim and a 3/4 in per ft facet at the near end, steep enough to check against your plumbing code. Run the pre-slope and the top bed separately.
Formula or tablerim thickness = drain thickness + pitch x the largest drain-to-wall distance (level perimeter); facet pitch toward each wall = (rim - drain) / that wall's distance; average thickness = (drain + 2 x rim) / 3; volume = length x width x average / 12; bags = ceil(volume / yield).
Source and editionIPC 2027 (International Plumbing Code). Section 421.5.2 (Section 417.5.2 in IPC 2021), read at codes.iccsafe.org on 2026-10-19: liners pitched 1/4 unit vertical in 12 units horizontal toward the drain and turned up not less than 2 in above the finished threshold. IPC 2027 sets no maximum shower-floor slope.
Free accessFree to read at codes.iccsafe.org.
What governsEstimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.
API field nameslength_ft, width_ft, drain_x_ft, drain_y_ft, drain_thickness_in, slope_in_per_ft, bag_yield_cuft, rim_thickness_in, average_thickness_in, volume_cuft, bags
- Level perimeter the rim is screeded level and each facet is planar from the drain to its wallpre-slope screeding practice
- Bag yield entered about 0.45 cu ft per 60 lb bag of sand topping or deck mud; use the bag's printed yieldmanufacturer data